We present a real-time formalism enabling us to compute statistical averages of various quantum string field correlation functions and scattering amplitudes in the context of perturbation theory and functional methods. Second quantization of the string fields is carried out in the path-integral representation. Our finite-temperature formalism is a generalization of thermo field dynamics to string field theory for which a doubling of the physical string field degrees of freedom is required. Temperature formally enters our construction by imposing boundary conditions (Kubo-Martin-Schwinger condition) in the generating functional for the string field. Some explicit computations for the case of covariant open bosonic string theory are carried out.